


Frigid Waterfalls

by lucadumpster



Series: Stay By My Side [2]
Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Anal Fingering, Anal Sex, Blow Jobs, Established Relationship, Exhibitionism, Fluff, Fluff and Smut, I hate tagging, M/M, Outdoor Sex, Porn With Plot, Porn with Feelings, Possessive Behavior, Senku is gaining more confidence, Smut, compliments, sex in the water ??, slight orgasm denial/delay
Language: English
Status: Completed
Published: 2021-03-09
Updated: 2021-03-09
Packaged: 2021-03-16 04:34:26
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,765
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29944551
Author URL: https://archiveofourown.org/users/lucadumpster/pseuds/lucadumpster
Summary: Senku hasn’t gotten much alone time with a certain mentalist and its driving him crazy.Luckily, the scientist has a lull in his schedule and decides to surprise his boyfriend.
Relationships: Asagiri Gen & Ishigami Senkuu, Asagiri Gen/Ishigami Senkuu
Series: Stay By My Side [2]
Series URL: https://archiveofourown.org/series/2202252
Comments: 4
Kudos: 83





	Frigid Waterfalls

**Author's Note:**

> I told you there would be a part 2 >:-)  
> Again, I apologize if it doesn’t match up w the manga/anime.

Being in a relationship with Gen was surprisingly easy for Senku. There was never a time where they had to clarify they were an item. It just happened. That night in the observatory had been enough for both Senku and Gen to know that they were more than just friends or — at the very least — a leader and his right hand man. The scientist felt they were a lot like a chemistry project: combining two completely different substances to create something stronger and quite possibly more useful in the grand scheme of things.

Senku still felt his heart race, adrenaline mixing into his bloodstream and knotting in his stomach, whenever he thought about Gen and what they had shared. Senku didn’t like physical contact, especially while sleeping, but he had to admit that he did sleep ten billion percent better holding the mentalist that night on a pallet of blankets and pillows on the observatory floor. So much so, in fact, that he requested for Gen to stay with him night after night until it was fairly common knowledge that they lived together. 

The only issue, though, was they hadn’t gotten much alone time aside from resting together and Senku was feeling antsy, like being unable to figure out an experiment’s outcome no matter what he tried. He had been devoting most of his time to science and the development of the village and no one else was aware of their very-new relationship; intimate relations with the mentalist put on the back burner. Which drove him more crazy than he expected. He couldn’t keep on with prolonged eye contact over the campfire and sensual brushes of fingers against skin in passing and nights where heated kisses turned into grinding, until Chrome began rustling around in the room below them. Senku needed to satisfy his building lust for his partner. 

Things were beginning to look up, though, as Senku finally saw a break in his busy schedule. His recently concocted plan would work, too, since it was still warm enough outside. The scientist shook his head and laughed silently. He was turning into a real softie, wasn’t he? 

“What’re you smiling at?” The mentalist was sitting on a stool across from Senku, his chin resting in the cradle he created with his interlocking fingers, elbows propped on the surface. A curious grin painted his face, an eyebrow cocked upwards. ‘He’s going to make me lose it if he keeps looking at me like that,’ Senku thought, his own smile unwavering. 

“I’m smiling because I’m almost done with my work for today,” Senku said, jotting down the last count of his inventory check. 

Gen’s eyes widened, the dark blue of his eyes standing out against the whites, his long eyelashes fluttering in excitement. “You mean we-.”

“-can spend some time together? Just the two of us? Yes, my mentalist, we can,” Senku said, completing Gen’s thought, his grin growing broader. 

Senku’s face began to flush as Gen’s smile started to beam, reminding him of the tungsten Suika found; the stone glowing blue in the first rays of morning. Even though they were a couple and slept together every night, the mentalist still had the tendency to drive Senku mental. But now, the scientist reveled in it instead of running away. 

“Since it’s still relatively early, why don’t we take a trip to the waterfall Kohaku and Chrome discovered last week?” Senku already knew Gen’s answer, but decided to ask instead of order. He really was going soft. 

Gen could hardly contain his excitement as he wiggled and twisted on his stool, hugging himself with both arms. The scientist enjoyed seeing this unfiltered version of the mentalist. It really showed how much they trusted each other at this point. Senku reached out a hand, brushing a thumb across Gen’s plump bottom lip and smiled when the mentalist stopped moving, his own cheeks staining red. 

“What’re you going to do when they find out about us?” he asked, his face still flushed and voice barely above a whisper. 

The scientist shrugged. “We’ll cross that bridge when we get there.” 

He was about to lean in to peck Gen on that soft-looking mouth when they both heard someone enter the observatory, making their noisy way up the creaky stairs. Both men looked at each other, eyes wide in a panic. They pulled away and settled themselves at an appropriate distance as to not out themselves. 

“SEEEENNKKUUUU!!” Chrome yelled as he reached the top of the stairs, running towards the scientist at ten billion miles per hour. He wrapped his arms around said man, almost toppling them both off the stool. 

“Get off of me. You’re being gross,” Senku snarled, trying to push the hyperactive former sorcerer off of him. He finally wrenched free, dramatically breathing heavy, as if he were just attacked by a bear. 

Thankfully, Chrome had been absent the night Senku and Gen had sex the first time, so the berage of embarrassing questions never came. The scientist had his own suspicions of Chrome’s whereabouts, but refrained from asking since it might draw attention to Senku’s private love life. 

“The mentalist and I are leaving for the afternoon,” Senku informed Chrome as they both stood up. Out of the corner of his eye, Senku could see Gen chewing at his lip, eyes wandering the straw roof aimlessly. He looked a tad uneasy, trying his hardest to be patient. 

“We should be back by dinner, so don’t wait up,” Senku said, walking away and lifting his hand up as a farewell to Chrome. 

The other scientist raised his eyebrows and cocked his head to the side, brown hair bouncing with the movement. “Alright, alright. Have fun, you two,” he said. If he wasn’t mistaken, Senku detected a knowing grin on his friend’s face. Chrome didn’t seem to be the type of person to notice things quickly, but maybe the scientist underestimated him a bit. 

Before any questions could be thrown their way, Senku and Gen hurriedly left the observatory.

🧪 • 🧪 • 🧪 • 🧪 

“Are we almost there?” Gen whined. “My feet hurt, Senku-chan.” 

Normally, Senku would’ve been irritated by his complaining, but it was another thing that the scientist had learned to love about the mentalist. He hated to admit it, but the way Gen’s voice got higher at the end of complaints was very endearing.

“Yes, mentalist. It should be just past this clearing. And haven’t I told you to wear shoes before? You’re going to get yourself hurt one of these times,” the scientist huffed. They were in the woods, passing trees and shrubs and the occasional stone-petrified human on the way to their destination. 

Gen trotted up behind Senku, feet disturbing rotting leaves on the forest floor. His hand grasped the scientist’s, lacing their fingers together as they now walked side by side. Senku felt his whole body react to the touch, his heart starting to race the way it only did when it came to the man beside him and science. 

“You do care,” Gen said, smiling up at Senku. 

The scientist was now feeling flustered, but he didn’t dare let go of the mentalist’s soft hand. Instead, he grumbled, “Of course I care, idiot.” 

The thunderous hum of rushing water hit both of them and before they knew it, they were standing next to a natural pool, a decent sized waterfall cascading from the rocks above. 

“It’s so beautiful, Senku-chan!” Gen exclaimed, releasing the scientist’s hand and running towards the water. “Can we get in?”

Senku nodded and Gen was already stripping off his clothes. With a smirk, Senku thought, ‘He has no shame.’ 

The mentalist dipped a toe into the pool and quickly recoiled, exclaiming, “Holy shit, that’s cold!”

“Yes, well, it’s ten billion percent coming from upstream, so it’s obviously going to be colder,” Senku said, shaking his head at what should be common knowledge. 

Gen glowered at the scientist as he said, “Not all of us are as smart as you.” 

“You’re plenty smart, my mentalist. I wouldn’t have been able to fall in love with you if you weren’t.” 

Gen looked over at Senku and beamed, all of his straight white teeth gleaming. The sun shined around his head like a molten halo, rendering the white half of his hair a muted gold. A slight breeze picked up and his long fringe flowed gently behind him. The scientist didn’t believe in magic and nonsensical things, but he swore time slowed at that moment.

“You’re really pretty,” Senku blurted. 

“Wow, Senku-chan. Two compliments back to back. Did you hit your head?” The mentalist teased. Before Senku could retort, Gen plunged into the water, his head sinking below the surface. 

The scientist sighed and began taking his clothes off slowly, facing away from the water. It was true that Senku very seldomly handed out compliments, but was it really that big of a deal? Gen was pretty. To be more precise, he was beautiful. He had been famous in the pre-Stone Age and he could distinctly remember other classmates gushing over his soft face, long eyelashes, shiny hair, and slender frame. Senku couldn’t be jealous, though. That was so far in the past that it wasn’t relevant anymore and in the end, Gen had fallen for Senku. 

Completely nude now, the scientist went to return to the water, but a pair of wet arms wrapped around his middle, damp lips dusting kisses along his right scapula. His heart rate quickened again, his skin prickling with goosebumps. 

“Come join me, my dear,” Gen hummed, “I’ve missed you.” 

“I’m always with you.” The mentalist couldn’t see Senku’s face from his angle, but the scientist had his eyes closed, trying to calm his erratic breathing pattern. For the ten billionth time, Senku wondered how another human could make him feel like this. 

“You know what I mean,” Gen’s hot breath whispered against his ear. 

Senku chuckled as the mentalist led him to the water, both of them stepping in. There was a rock ledge on the side that made the pool easy to access. Senku hissed at the sudden shock the frigid water gave him and settled to sit on the ledge, the water to his waist. Gen sat beside him, pulling his legs to his chest and rested his head against his knees, looking over at Senku. 

“You don’t give yourself enough credit, Senku-chan,” Gen said. “I know you don’t believe in superficial stuff, but I think you’re really pretty, too.” 

Gen had said Senku was beautiful that night, too, as he gazed upon the scientist’s naked body, but, Senku wasn’t sure if it was just the sex talking. He had never really felt exceptional in the looks department before. Even when Kohaku complimented him all that time ago in the river, trying to make magnets, it just felt like something to say, not the actual truth. 

But, Senku wasn’t one for drama in the slightest. Instead, he asked, “You really think so?”

“I know so,” Gen said, a flicker of determination in his eyes that Senku had never seen before and took an immediate liking to. The mentalist rubbed a hand across Senku’s newly-developed abs, snaking his way up his chest and across to his biceps and squeezed. 

“You’ve gotten stronger since being at the village. You have a sharp jawline and smooth skin. Full lips and uniquely colored eyes. I’ve never seen red eyes like yours before.” Gen’s voice was hushed as his fingers brushed every area he complimented, lingering on the skin beneath Senku’s eye. 

“Th-thanks. No one’s ever been so specific before,” Senku mumbled, looking down at his exposed body. He contemplated his next words and began speaking in a hushed tone.

“I… I really like when you smile genuinely. It makes you look really happy. And… I like how easily you blush. I like how small your waist is because it’s… easy to hold onto. I like the way you mumble in your sleep. I like your voice and the sound of your laugh… I like your tiny hands and smooth legs… I’m really bad at giving compliments, huh?” 

Senku looked over and his face fell, seeing tears slipping their way down Gen’s cheeks. If he was bad at giving compliments, he was terrible at consoling people. 

“Did I say something wrong-?”

“Senkuuu-chaaaaan!” Gen wailed, throwing his arms around the scientist. “You’ve never said such nice things before! You really think all of that?”

“Well, duh. I wouldn’t just say it if I didn’t believe it,” Senku said, lifting a tentative hand up and wiping the remainder of Gen’s tears away. 

“Let’s go behind the waterfall. I want to see if there’s anything exciting back there,” Gen said suddenly, taking Senku’s hand and drifting towards the other side of the pool. The mentalist turned around with a mischievous twinkle in his eyes and pushed the scientist under the waterfall. Senku spluttered and pulled Gen with him by his wrist, drenching them both in freezing water. They stepped out, Senku’s usually gravity defying hair flat against his head, the wavy locks of green and white falling to his shoulders. 

“Are you happy now?” Senku asked, clearly irritated, hands on his hips. 

“Wow,” Gen whispered, subduing his giggles from his prank, reaching a hand up to comb his fingers through the scientist’s hair, marveling at what happened when the scientist’s hair got wet. 

“Before you ask, no, it won’t stay like this. I stuck my finger in a light socket as a kid and no matter what I do, it always stands right back up when I dry it,” Senku huffed. 

“No need to be upset, dear,” Gen said, clasping his fingers behind the scientist’s neck and leaning forward. “I like your hair either way.” 

Senku closed his eyes and pressed his lips to Gen’s, parting them to lick inside the mentalist’s mouth. There was something thrilling about the taste of his tongue, something foreign and delectable. It should’ve just tasted like spit, but it was more complicated than that. 

As if reading his mind, Gen murmured, “You taste good.” 

They continued to kiss, the mentalist backing Senku up against the edge, motioning for him to sit up there. 

“I want you in my mouth this time.” 

Senku’s entire body flushed hearing those words, his brain flaring with pleasure even though they hadn’t even done anything yet except kiss. He lifted himself up to sit exposed upon the rock ledge, waiting in anticipation at Gen’s next move. 

He settled in the water in front of the scientist, kissing the inner side of his thighs and grazing his sharp canines against the soft flesh. Senku sucked in a breath through his teeth and ran a hand through his wet hair, sending droplets cascading over both men. Gen licked a line from the base of Senku’s erection, up the shaft, and wrapped his lips around the head. If that wasn’t already driving Senku up a wall, the mentalist giggled through his nose and went all the way down, gobbling it all up in one go. 

“Jesus fucking Christ,” Senku exclaimed, all his muscles tensing, feeling his abdomen clench up. 

Gen immediately removed his mouth, tutting and shaking a finger. “Nuh-uh. You can’t cum yet, Senku-chan.” 

The scientist regained some of his stamina, breathing heavy, sure that his face was more red than his eyes. Once he had recovered, Gen lowered his head again, sucking on the tip, each stroke going lower and lower. His tongue had a life of its own, curling and twirling its way around. His moans reverberated against his cock, adding to the cacophony of sensations he was casting on his scientist’s body. Senku raked his hands through Gen’s black and white hair, encouraging him to keep going. 

“Ahh — shit, Gen… this i-is exhila-,” the scientist moaned before two fingers were shoved into his mouth. Gen’s hands were exactly as soft as they looked. They tasted salty from natural skin secretions and river water, with a floral aftertaste. Even when he wasn’t carrying black nightshade on him, he still faintly smelled and tasted of them. Imagine: a man who smelled like flowers in the Stone Age. Senku had really found a needle in a haystack. 

The scientist stifled his moans as he licked and sucked gently on Gen’s fingers, whimpering unabashedly when they were pulled free. He looked down at his mentalist who easily slid up and down his shaft, rolling his eyes back and mewling. Every time Senku had attempted to watch porn, he always felt the people giving head looked... odd. The blowjob part of the videos were always skipped because he could never get off to it. It made him feel awkward, thinking about some stranger’s mouth around his dick. It wasn’t even a millimeter attractive to him and it had always been that way.

But, Gen…

Gen was a completely and utterly different story. 

“Mm..., Gen,” he moaned, thrusting his hips to stick himself deeper into his mentalist’s mouth, feeling himself hit that back of Gen’s throat. 

The mentalist gagged and coughed, lifting his head. And by all the scientists and engineers Senku had sworn to, it was the most gorgeous thing Senku had ever seen in his life. His face was flushed, eyes watery and half-lidded, drool and pre-cum stuck itself to his lips, chin, and cheeks. He burst into a fit of laughter and said between giggles, “You should see your face right now.”

“You’re one to talk,” Senku said, out of breath. “I want to fuck you so bad right now that I feel like I’m dying.” 

Gen’s eyes flashed and he arose from the water, exposing his own hard on, water droplets running in rivulets down his thin frame.

“Good thing I stretched myself for you,” the mentalist said with a wink, removing three fingers from between his well-developed behind.

Senku gulped as he realized Gen had been using Senku’s saliva to finger himself with. No matter how he tried to look at it as disgusting, he couldn’t. Thinking about a part of his DNA (no matter how miniscule) going inside his mentalist was exhilarating. 

“I wanna make you feel real good, Senku-chan,” Gen slurred, positioning himself over the scientist’s devoted and twitching cock. Senku shuddered with pleasure as the mentalist pressed down, allowing Senku to enter. 

But, it wouldn’t budge any further than the rim of Senku’s tip. 

“Damn it,” the scientist hissed, “I forgot lubricant.” How were they supposed to do this now?

“Don’t worry, my scientist,” Gen purred. “Hold out your hand.” 

Senku did as he was told, heart fluttering. Gen bowed his head and let a string of saliva fall from his lips and tongue into the scientist’s open palm. It was warm and clear, reminiscent of lube and coconut oil when you rubbed your fingers together. Would this really be okay to use? 

‘If it’s Gen’s, I don’t mind,’ Senku thought as he slicked his cock with the saliva, giving it a couple strokes, breath heaving in excitement. 

Again, the mentalist lowered himself onto Senku, the scientist’s cock slipping in with less resistance. He missed this feeling greatly, his throbbing cock inside Gen’s terribly hot body. It was ridiculous how amazing it felt as the mentalist began to move up and down, his hips grinding against Senku’s trembling thighs. 

“Nghh…,” Senku groaned, “You’re unbelievably tight.” 

“Does it feel good? Do I make you feel good, Senku-chan?” Gen moaned, a desperate tone to his lustful voice. 

“Do I — uhn! — really need to clarify?” Senku said, his voice just as full of lust. “I think it’s fairly obvious how you — ah, fuck — make me feel.” 

Gen pressed himself all the way down, taking the entirety of Senku’s length, both of them exclaiming loudly in pleasure, throwing their heads back. 

“Humor me, scientist.” 

Senku couldn’t take it anymore. He wanted — no, needed — to be in control. With an almost animalistic growl, he flipped them over so the mentalist was on his back and Senku above him, a hand gripping Gen’s slender hip and the other behind his head. It sent a thrill up the scientist’s spine at how similar the position was to the first time they made love. 

“You make me feel so good that my very atoms begin to split apart and reform at the speed of light, Gen. I feel like I could destroy everything and create everything at once. That’s how good you make me feel,” Senku said as he rocked himself in and out of Gen, speeding up. 

“Ahh! Senku, Senku, please,” Gen squealed as he flailed his arms about, the pleasure making him erratic. 

“I tend to like things in an orderly fashion,” Senku muttered, a smirk across his face. “But, I’ve learned that I love to make a mess of you.” 

Gen wrapped his pale legs around his scientist’s hips, his hands clutching around Senku’s shoulders, begging with his body for Senku to fuck him harder. 

“I’m so close — mmh!” The mentalist cried out, dragging his perfectly manicured nails across Senku’s skin. It would definitely leave marks, but the scientist couldn’t care less. In fact, he thought it was pretty sexy that his boyfriend left scratches on him; it meant he was doing a good job. 

Senku began lapping at Gen’s neck, then sucking, leaving bright red welts in his wake. So what if anyone saw (even though the mentalist’s shirt collar was absurdly high)? Gen was his. 

“You’re mine,” Senku rasped. “You’re mine, Gen. Only mine.” Even he had to admit his voice sounded completely different than usual, a hunger and possessiveness simmering in his tone, setting his brain alight. 

“I love you,” the man under Senku whimpered, his body tensing and shuddering as cum shot out of him, landing on both of their chests. “I love you so much. Please, cum in me.”

That was enough to set the world on fire. 

Senku cried out, his voice breaking as he painted Gen’s insides. He rode out his orgasm, trembling slightly. The scientist just couldn’t get over a substance he produced entering his love; just couldn’t get over the thrill of something biologically his mingling with something so precious to him.

He pulled out, both of them sighing in contentment. Senku planted a kiss on Gen’s jagged face crack and rested his forehead there. 

“We should probably head back soon,” the scientist whispered. “But, before that, let’s get cleaned up.”

“Way ahead of you, Senku-chan,” Gen laughed as he caressed the side of the scientist’s face, smiling in that way that made Senku believe in miracles.

**Author's Note:**

> I hope you enjoyed!! I’m planning on publishing more parts to this because I love writing about their developing relationship :,) 
> 
> Kudos and respectful comments are always appreciated!!
> 
> My [Twitter](https://mobile.twitter.com/)


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